Hollow-cast cast parts with cooling passage arrangements inside the walls refer within the spirit of the invention primarily to components which are to be integrated into gas and steam turbine plants and are exposed to high process temperatures for service-induced reasons and require effective cooling for avoiding thermally induced material degradations. Especially stator blades and rotor blades within turbine stages, which are directly exposed to the hot gases of a gas turbine process, constitute such cast parts. As a rule, the cooling of such blading arrangements is carried out by means of cooling air which is tapped off on the compressor side and fed via openings inside the respective blade roots into the blade airfoils, which have cavities, for cooling purposes.
For illustration of the previously applied cooling technique of stator blades for use in gas turbine plants reference may be made to FIGS. 2a and b which show a known per se stator blade with a stator-blade platform 1 and also a stator-blade shroud 2, between which extends the stator-blade airfoil 3 with a stator-blade leading edge 4 and a stator-blade trailing edge 5. For cooling the stator blade 3, formed hollow inside, which is shown partially cut away in FIG. 2a for illustrating the inner hollow cooling passage arrangement, cooling air K finds its way both through openings inside the stator-blade shroud 2 and inside the stator-blade platform 1. For effective cooling of the stator-blade airfoil 3, in the interior of the stator blade there are flow contours which ensure a thermal contact which is as intimate as possible between the supplied cooling air and the inner side, which is to be cooled, of the stator-blade wall. In particular, in the flow region directly upstream to the trailing edge 5, which is shown enlarged in FIG. 2b, there are rib lines 6, extending in the flow direction, which delimit individual cooling passages 7 from each other in each case. The rib lines 6, which are oriented parallel to each other, are connected in each case on both sides to the oppositely disposed stator-blade inner walls and therefore close off two directly adjacent cooling passages 7 from each other. For improving the cooling effect in this flow region, provision is made along the cooling passages 7 for a large number of individual peg-like connecting lands, so-called pins 8, between the spaced-apart oppositely disposed inner sides of the stator-blade walls, as a result of which cooling air experiences an effective mixing-through and therefore comes into intimate contact with the inner sides of the stator-blade walls.
For producing such filigrane cooling structures inside a stator blade or rotor blade which is to be produced by way of a casting process, so-called lost cores are required for the casting process, in which core the negative contours of all the structures which are to be provided inside the cast part, especially the flow contours which influence the cooling air flow, are to be incorporated. In order to form for example the rib lines 6 which are shown in the detailed view according to FIG. 2b and also the peg-like pins 8, which for better illustration are shown again in FIG. 3a in a plan view, it is necessary to provide a casting core 9, similarly shown in FIG. 3b in plan view, which has to be provided for creating the individual rib lines via groove-like recesses 10 and for creating through-holes 11 corresponding to the peg-like pins 8. The entirety of all the recesses which are to be provided inside the casting core 9 lead eventually to extensive perforation of the casting core and contributes decisively towards mechanical weakening of the casting core so that ultimately mechanical stability limits are reached and exceeded, these limits no longer allowing a damage-free machining and ultimately the forming of the extremely small flow contours inside the cast part. In order to stabilize the casting core, modifications have been undertaken especially during the forming of the previously described rib lines so that the casting core provides connecting lands 12, which stabilize the casting core, transversely to the longitudinal extent of the respective rib lines. As a result of this measure, however, the rib lines 6 are no longer formed continuously in the finished-cast cast part, as is to be gathered from the view in FIG. 4, but where the connecting lands 12 were provided in the casting core now have corresponding gaps 13 (see FIG. 4b).
If previously continuously formed rib lines 6 were able to completely separate the cooling air flows K contained inside the cooling passages 7 from each other, as is shown in the schematized plan view in FIG. 4a, then by providing corresponding gaps 13 along the rib lines 6, attributable to the stabilizing connecting lands 12 inside the casting core, cooling air flows K′, which branch off through the gaps 13, now occur and are able to irritate the cooling air flow in the adjacent cooling passages. This, however, reduces the cooling efficiency of the cooling air which passes through the cooling passages 7 so that measures have to be sought with which the cooling air flow portions which pass through the gaps 13 can be avoided.